As long as your subs have their own amps, using the front speaker outs to feed it signal should not be a problem. If they don't have their own amps, then you could still use a receiver with RCA out, you would simply need to have an amp to feed that signal to that will power the sub. Yes that may cost about as much as just getting a new sub that has line level (RCA) input. Center speaker out won't work because it only carries the center speaker's information.